A MMC is when the baby stops developing and the body continues to keep the pregnancy, or the baby inside.
It's usually found out later on during a scan, and needs medical assistance to remove the baby. (D&C, cytcoc) Some women also wait until the body kicks in, which could take weeks.
My second MC, I assume was also a MMC, I started spotting at 6+4 and went to the doctor where we found out the baby had stopped developing at 5 weeks. I later miscarried naturally that evening. I consider it though as a MC, because I never knew before the MC actually started happening that the baby wasn't growing.
With my first we saw a healthy baby with a HB at 6+4, but at 8 weeks we discovered the baby had stopped developing. I had a D&C the next week, my body made no signs of miscarrying.
A chemical pregnancy usually happens between when AF should arrive until 5 weeks. Before a baby can be detected on US. Some women prefer to call it an early loss/MC.
A chemical cannot happen before AF. Early bleeding is not a chemical, probably just an early AF.